strange universal dock problem

I'm running a 3G with universal dock a griffin car charger, and a standard 3.5mm to 3.5mm cable with the prospect of modding my car dash to allow for the dock to permanently be there. My problem; when i play music through my stereo it plays the first split second of the song and then cuts out, skip track the same thing happens etc etc...I've also tried plugging it into some standard computer speakers and it does the same thing!!!